Hydraulic fracturing is a technique for increasing the output and productivity of oil and gas wells by cracking the geological formation surrounding and defining an oil and/or gas reserve to create pathways through which the entrapped oil and/or gas may more easily flow for extraction. First, a highly pressurized fluid is injected into an existing well bore at a sufficiently high rate of flow to put sufficient stress on the geological formation to induce fracturing thereof, thus creating a network of cracks in the rock defining the oil and gas reservoir. Next, a fluid containing a vast amount of small particulate propping agents, or proppants, is introduced into the crack network such that the proppants will become positioned in the newly-opened fissures to prevent their closure due to geological forces. In other words, the proppants literally “prop” the cracks open.
To do their jobs, the proppants are typically formed to have sufficient mechanical strength to hold the cracks open against the dynamic geological forces that would otherwise operate to close or distort them. Typically, these geological forces increase with the depth of the well. Also, proppants are typically made to be somewhat fluid permeable and/or conductive, such that even when present in aggregate, they do not substantially obstruct the flow of oil and/or gas desired to be extracted from the well. Typically, propping agents have been made stronger through densification or by increasing the alumina content thereof. However, denser, heavier proppants are harder to pump, more expensive to transport, and are less permeable than lighter, more porous agents.
Another desired characteristic of proppants is that they be inexpensive to produce, since it takes a great volume of proppants to hold open cracks in even a relatively small well. Sand is cheap and plentiful and is often selected as an advantageous propping agent for maintaining the cracks formed in wells and geological formations experiencing relatively low closure forces (i.e., 4,000 psi or less). Moreover, the strength of the sand may be extended to withstand closure forces of 8000 psi or more through such sorting processes as screening, sizing and shaping the sand. However, the sand proppant performance drops off dramatically as the closure forces increase, such that even highly processed and selected sand is inadequate under closure forces much exceeding 10,000 psi. Further, sand tends to be nonporous, and as such is less than ideal from a permeability standpoint. Moreover, the sorting and processing steps add expense, thus detracting from one of the main characteristics, low expense, making sand attractive in the first place.
Some high-alumina aluminosilicate compositions, such as bauxite with an alumina content in the 75-90% range, offer sufficient strength to function as proppants under relatively high closure forces and at relatively great well depths. However, these high-alumina proppants likewise have high densities/apparent specific gravities approaching or exceeding 3.5 g/cc, and thus add the requirement of high viscosity pumping fluids and/or high pumping rates to prevent them from settling out during the injection process. Increased fluid viscosity and the requisite high pumping rates cost precision and control of the injection operation, thus making fracture control and high conductivity fractures more difficult to achieve and maintain. Moreover, the high-alumina proppants tend to be more abrasive, and thus speed the wear of the pumping and fluid transport equipment. Additionally, sintered high-alumina compositions are relatively expensive, often priced ten to fifteen times that of sand.
Intermediate density proppants, defined as those having an apparent specific gravity in the 3.1 to 3.4 g/cc range, have been developed to provide sufficient strength to keep cracks open at well depths of from about 8,000 to about 12,000 feet. In these materials, lower density is achieved primarily by reduction of the alumina content to about 75%. Proppants having even lower densities, such as around 3.0 g/cc, have been formed from kaolin clay precursors and are characterized by an alumina content of about 50%. These low density proppants are typically intended for use at well depths up to about 8,000 feet.
An even lower density proppant has been developed having an alumina content of from 25% to 40% and an apparent specific gravity of from 2.20 to 2.60 g/cc. While the reduced density allows for the use of less viscous pumping fluid and lower pumping rates (which are both desirable for prolonging equipment life and thus reducing repair and replacement costs), the tradeoff is in proppant strength. Lowering the alumina content of the material generally results in a lower density proppant with corresponding lower strength, since the higher silica content results in significant loss of strength.
Accordingly, there has been a definite need for a proppant composition enjoying both lower density and less expensive precursor materials that also has yield proppants having sufficient mechanical strength to withstand closure pressures of 8000-10,000 psi or greater. The claimed novel technology addresses these needs.